1. Field of the Invention
The present invention relates to the biaxial orientation of a deformable sheet material such as that of precrystalline polymers and, more particularly, to the biaxial orientation of polyacrylonitrile.
Polyacrylonitrile has been known and available since the late 1940's, but has not been available in a commercially useable form until relatively recently because of its intractibility. Basically, polyacrylonitrile (hereinafter referred to as P.A.N.) cannot be melted without decomposition of its molecular structure, therefore rendering conventional hot melt extrusion techniques inapplicable.
2. Discussion of the Prior Art
Shaped articles of polyacrylonitrile have been described in prior patents. See, e.g. U.S. Pat. Nos. 2,858,290 and 3,094,502, which disclose polymerizing acrylonitrile in dimethyl sulfoxide to provide a spinning solution of polyacrylonitrile in the foregoing solvent, followed by extrusion. Reference may also be had to U.S. Pat. No. 3,437,717 which describes the preparation of a film made from a copolymer of acrylonitrile and a polyalkenyl monomer, the film being prepared by dissolving the copolymer in a polar organic solvent such as dimethyl formamide or mixtures of dimethyl formamide with dimethyl sulfoxide, followed by casting the film from the foregoing solution, removing part of the solvent, stretching the film biaxially, and removing most of the remaining solvent by air drying.
In U.S. Pat. No. 4,066,731, there is disclosed a method of preparing acrylonitrile homopolymer films which are characterized by very high tensile strength, stiffness, optical clarity, and outstanding gas barrier properties. Production according to the breakthrough process taught therein involves, inter alia, applying a hot, concentrated solution of the acrylonitrile in a suitable solvent which is also water-soluble, onto a film-supporting surface such as a cooled, flat surface or a cooled, rotating drum. Dimethyl sulfoxide is indicated as a typical solvent that is suitable for this process. The process described in the '731 patent also envisages contacting the cast film with water to remove the solvent while under constrained conditions to prevent shrinkage, and, possibly even while undergoing biaxial orientation.
In addition to the higher solvent-removal effect of exposing the film to water under restraint, this type of water treatment results in a film composed of polyacrylonitrile and water which, ostensibly, has entered the interstices and voids present in the film on a molecular scale so that the water is an integral part of the film.
Such film, by virtue of its high water content, is highly resistant to ignition. This film has a wide variety of end use applications. For example, such a film is highly suitable as a plate separator for batteries, as a semi-permeable membrane for use in dialysis or reverse osmosis, and so forth.
Moreover, if the cast film in the above process is subjected to biaxial stretching (as opposed to mere restraint) in a moist environment, for instance, in the presence of steam, moist gas, or in hot water, a final film is produced which exhibits extraordinarily high tensile strength and extra-ordinarily low gas permeability.
The biaxial stretching can be carried out in a one-step operation wherein the film is simultaneously subjected to stretching in two directions which extend at right angles to each other. Alternatively, the biaxial stretching can be carried out in stepwise manner, such as initially by longitudinal stretching and thereafter by lateral stretching, or conversely.
One method for effecting biaxial stretching simultaneously in both directions is to clamp the film between two plates, one plate having a large circular hole therein and the other plate having a small hole for the introduction of compressed air. The foregoing assembly is then immersed in hot water. At a water temperature of from at least 40.degree. up to about 100.degree. C., compressed air is admitted under sufficient pressure so as to cause the film to expand through the large hole to thereby form a "bubble." Within that temperature range, it is highly preferable that the higher the temperature the more promptly should the air admittal be initiated. For example, at about 70.degree. C., the air admitted is desirably substantially immediate. The degree of stretch is, of course, determined by the size of the ultimate bubble. After a bubble of desired size has been formed, and while maintaining the air pressure so as to retain the bubble, the entire assembly may then be immersed in cold water so as to "fix" the bubble structure. The bubble portion may then be cut away (this being the portion that was subjected to true biaxial stretching) and then dried while under restraint, for instance, in an oven. During the drying, the film, which initially exhibited the spherical curvature defining the bubble, will shrink and become flat as it loses its water. The resultant flat film is dimensionally stable and has very high tensile strength and extremely low gas permeability. The bubble method, however, suffers certain disadvantages such as disproportionate biaxial orientation.
Rather than the foregoing "bubble" technique, other methods well known in the art may also be used. Thus, the biaxially oriented films embodied herein may, for example, be produced by sequentially orienting the film as, for example, by stretching it in a longitudinal direction (direction of feed) followed by stretching in a lateral direction (transverse direction), or by subjecting the film to orientation simultaneously in both the longitudinal and transverse direction. Specific apparatus and techniques for effecting biaxial orientation of the film embodied herein include, for example, the drafter-tenter frame arrangement as disclosed in U.S. Pat. No. 3,437,717 for sequentially imparting longitudinal feed direction and transverse direction orientation; and the various arrangements disclosed in, for example, the articles by J. Jack in British Plastics, June 1961, pages 312-317 and July 1961, pages 391-394, for sequential or simultaneous biaxial orientation of plastic films.
Irrespective of the employed method of stretching, it must be relatively rapid and efficient inasmuch as stretched film which is exposed to high temperatures for a period of time as short as two minutes, even in an aqueous atmosphere, will suffer a significant loss of water content, thereby causing difficulty in continuously orienting the film without its breaking. Thus to achieve smooth stretching and an attractive useable final product, the "wet" film, (in effect, film composed of polyacrylonitrile and water) should be quickly heated while preventing substantial evaporation from occurring, and then stretching the film immediately before the film can adjust to its new environment and lose much of its water.